1. Field
The present disclosure relates to an optical receiver, and in particular, to an optical receiver that provides improved equalization.
2. Related Art
At the receiving end of an optical link, an optical signal (i.e., light) is usually converted into an analog electrical current by a photo-detector in an optical receiver. The optical receiver often includes a transimpedance amplifier (TIA) to convert and amplify this current into a voltage, which can then be restored to a digital signal level. At high speeds, TIA designs typically require high power to achieve reasonable gain and acceptable noise performance.
At a high data rate, the main cause of optical receiver failure is usually inter-symbol interference (ISI). This means that the communication channel cannot respond to rapid changes in the optical signal and the channel smears a single bit over several bit periods. This effect is illustrated in FIG. 1, which illustrates sampling of an analog electrical signal with equal sampling periods. In this case, an impulse (a single ‘1’ surrounded by ‘0s’) is transmitted across the communication channel. The received waveform for this impulse is called the ‘impulse response.’ This particular impulse response has a peak at t=2, which is henceforth referred to as the main cursor. Because of communication-channel degradation, the impulse response also has nonzero samples for roughly five clock cycles after the main cursor. These nonzero samples after the main cursor are henceforth called ‘post-cursors.’
If a channel has an impulse response with several post cursors, then the post-cursors may interfere with future transmitted bits, making analog-to-digital recovery difficult. For example, FIG. 2 illustrates ISI for a ‘1010’ data pattern transmitted on the communication channel. Because of the post-cursors from the first transmitted ‘1,’ the ‘0’ in between the two ‘1’s has a rather large value, and it can be difficult to tell whether a ‘0’ or ‘1’ was transmitted.
Fortunately, for a deterministic communication channel, ISI is completely deterministic. In principle, if the impulse response of the communication channel is known, the resulting ISI can be corrected. For example, as illustrated in FIG. 3, a multi-tap feedback equalizer can be used to correct for ISI after the optical signal has been converted into an analog electrical signal. (In particular, each tap in the feedback equalizer may correspond to one of the post-cursors.) Exemplary waveforms are shown in FIG. 4. Note that the communication channel in FIG. 4 has the same impulse response as in FIG. 1. Moreover, note that the comparator and the four flip-flops in FIG. 4 all have an initial value of 0.
At t=2, corresponding to the peak of RXin, the comparator detects that a ‘1’ was transmitted. Using the known impulse response, it follows that there will be a post-cursor, with an amplitude of 0.25, at t=3. With this information, the optical receiver can then subtract 0.25 from the received waveform at t=3. Applying such correction for all the post-cursors (shown by the other feedback signals in FIG. 4) results in the analog electrical signal (RXin) having a perfect impulse response at the sampling points of the comparator. This feedback technique is known as decision feedback equalization (DFE).
DFE typically requires tracking of the history of the previous bits. For example, if a particular communication channel has twenty nonzero post-cursors, then the optical receiver may need to keep track of the past twenty bits in order to apply the corresponding feedback correction. In particular, if the communication channel acts as a first order low-pass filter, the number of post-cursors can be prohibitively large. However, if the feedback correction is applied to the input of the optical receiver (e.g., before the conversion to the analog electrical signal, such as at the output of the photo-detector), the number of previous bits needed can be significantly reduced. In the case of a first-order low-pass-filter communication channel, only a single previous bit may be needed for the feedback correction.
FIG. 5 illustrates an optical receiver with a DFE circuit that takes advantage of a first-order low-pass-filter communication channel. Because the impulse response of a first-order low-pass filter is an exponential decay, if the feedback-correction impulse at the input is properly scaled, the post-cursors associated with the decision-feedback-equalization feedback signal (DFEfb) may have the same magnitude as the post-cursors of the photo-current from the photo-diode (TXout) thereby perfectly canceling each other out. This is shown in FIGS. 6A and 6B. Consequently, the feedback correction may only need to be applied at the input once, and all the post-cursors can be eliminated.
In an optical link system, because the optical fiber or optical waveguide has much higher bandwidth than electrical wires, signal degradation typically occurs at the interface of the photo-detector and the electrical circuits, where there is a large capacitor due to bonding or wire traces. When combined with any resistance at this interface, this capacitor/resistor combination can be well approximated by a first-order low pass filter (which causes ISI). This input property of an optical receiver makes it a good candidate for applying the DFE technique.
However, because of delay in the optical receiver, there may be a gap between the end of the transmitted impulse response and the start of the feedback-correction impulse response. The effect of this delay is shown in FIG. 7. At t=3, the position of the first post-cursor in the voltage Vtx in RXin associated with TXout has not finished settling to its peak value yet. This causes incomplete cancelation of the first post-cursor by the voltage Vfb in RXin associated with the DFEfb, and can result in a degraded eye diagram and reduced optical-receiver sensitivity. Furthermore, because the post-cursor cancelation ability is limited by the optical-receiver delay, at higher data rates, when bits are more closely spaced, the feedback technique illustrated in FIGS. 6A and 6B may result in not just one, but multiple uncanceled post-cursors. This may impose a severe limit on the achievable data rate.
Hence, what are needed are an optical receiver and an equalization technique without the above-described problems.